The objective of this project is to further define the physiological factors which regulate fetal breathing movements in utero. The experimental animal will be the fetal lamb with the majority of studies performed in chronic unanesthetized preparations in which the fetus is monitored in utero. The contribution of the peripheral and of the central chemoreceptors will be examined. We will specifically examine the role of central hydrogen ion; the mechanism by which fetal hypoxia produces apnea; the effect of fetal oxygen tension on the activity of the carotid body and the role of beta-endorphin in fetal breathing. Fetal breathing movements have been proposed as an index of fetal well-being in human pregnancy. An understanding of the basic regulation of these movements is essential before proceeding to clinical application.